Tetrode Screen Control and G2DAF Amplifier Circuits

There was an interesting debate on the amps.contesting.com List Server about the application of dynamic screen potentials on tetrode and pentode rf amplifiers. The debate was centered around the G2DAF type design and similar circuits. Other methods to approach a dynamic screen supply were offered up by ZL1AAX, G2MA, ZL2AG and W6EDD. This file down load is a scan of an article from a 1958 edition of the New SSB Handbook (W6TNS).

On page 154 in the Linear Amp chapter, there is "A Class C Linear R-F Amplifier," a grid-driven 813 amp with a 6L6 screen clamp tube controlled by the grid-leak bias of the 813. The circuit was supplied by Lester Earnshaw ZL1AAX. This is followed by "the famous G2MA linear amplifier" which uses a thermionic diode to rectify RF grid voltage to control a 6Y6 clamp tube on the screen of an 813 grid-driven amp.

This is followed two pages later by "a grounded grid gated linear amplifier, designed by Norm Roller, W6EDD." It has parallel cathode-driven RK-65s, with a 6L6 rectifier/cathode-follower whose grid is driven by the incoming RF, and whose cathode output drives the screens of the RK-65s. This amp is claimed to run 2 kW input to the surplus RK-65 tubes. Then there is an Americanized version of the ZL linear, using an RCA 7094 with a 6L6 clamp tube.

Many thanks to Tim Bratton, K5RA for photo copies of these interesting circuits applied to "CLASS C Linear RF Amplifiers." The amps debate centered around the distortion products produced by operation of these circuits with voice modulated rf signals vs two-tone and steady carrier. The debate can be researched by searching through the amps archive at http://www.contesting.com/km9p/search.htm

It is my personal opinion these circuits produce unaccepted amounts of distortion when driven by complex (voice) radio frequencies. There are opposing opinions from other Amateurs who are welcome to provide supporting information and their examples here. I present this material as a reference and example of these circuits. In my opinion, fixed screen potentials are the way to go.


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